2GG 
MAJOR A. E. OXLEY ON THE INFLUENCE OF MOLECULAR 
spontaneous magnetization is concerned, but the tenacity might even surpass that of 
the chaotic /3-state since the transition /3—y is accompanied by shrinkage. Such an 
increase of tenacity was actually found by Rosenhain and Humfeey.* 
The eftects of prolonged heating on the plasticity of mild steel are interesting in 
this connexion. EwiNG and RosenhainI' have shown that the plasticity of a material 
is caused by slips occurring on cleavage or “ gliding ” surfaces within each of the 
crystalline grains, although the elementary portions of the crystals retain their 
primitive form and the crystalline structure of the metal as a whole is preserved. In 
the case of mild steel, exposure to a temperature of 1200° C. or higher temperatures 
for several hours may cause the material to lose much of its plasticity, while some 
specimens of soft iron after prolonged exposure at 700° C. to 800° C. (less than the 
critical temperature) have been made brittle. These results are consistent with a 
similarity of molecular configuration for temperatures above 1200° C. and below the 
critical temperature (about 850° C.). On the other hand an exposure at 900° C. or 
1000° C. (be., in the region of the paramagnetic state), followed l)y a slow or fairly 
rapid cooling, induces considerable plasticity in the material, and this treatment may 
even be used to remove brittleness originating from heating to the higher or lower 
ranges of temperature mentioned above. As the plasticity is produced by slipping on 
cleavage surfaces within the crystalline grains, this smallness of the grain structure 
may, under stress, determine a molecular rotation. Ewing and Rosenhain J have 
shown that in some metals, in addition to slips or motions of pure translation, there 
results a molecular rotation from strain which gives rise to twin-crystals. It is 
interesting to note that the formation of twin-crystals is common in iron through the 
y-range, but has not been observed in the /3 and a ranges. 
As the temperature falls below 1400° C. there is some modification of the crystalline 
cubic arrangement, resulting in a closer packing of the molecules, and accompanied by 
an interlocking of the fine grains. Thus iron in the y-range (1400° C. to 900° C.) will 
be paramagnetic. At a lower temperature than 900° C., this state is unstable and 
another modification of the crystalline grouping occurs, accompanied by expansion, 
thermal evolution, and the appearance of spontaneous magnetization. This latter effect 
seems inconsistent with a more open packing of the molecules, but an analogy is found 
in the case of water, where the molecular influence in the liquid just above 0° C. is 
small compared with that in ice just below freezing point, although the packing of 
the molecules in the liquid state is closer than that in the crystalline state (see supra 
p. 264). As the temperature is lowered the transformation progresses rapidly until 
a point B is reached, after which the increase of magnetic quality is somewhat less 
rapid. On continued cooling, the iron passes into the a-range where the magnetic 
property is capable of attaining a saturation value. 
* ‘ Roy. Soc. Proc.,’ A, vol. 83, 1909; ‘ Iron and Steel Institute,’ No. 1, 1913. 
t ‘ Phil. Trans. Roy. Soc.,’ A, vol. 193, p. 279. 
X See Ewing, ‘The Strength of Materials,’ p. 47, 190G. 
